The present invention relates to a direct current or DC machine and, particularly but not exclusively, to a DC motor.
DC machines may be used both as a motor (for producing movement) and as a generator (for generating power). Although the machine is hereinafter described with particular reference to its use as a DC motor, this is not intended to be a limitation of the invention, i.e. the machine may alternatively be used as a DC generator.
There are two well-known types of DC electric motors. In the first type, an electrical power source is connected by means of contact brushes to coils (windings) on an armature which is arranged to rotate about, or within, a stator in the form of one or more permanent magnets, or separately excited field magnets. The rotation of the armature relative to the brushes via a commutator also serves to switch electrical current between the coils. In the second type, the rotor comprises a plurality of permanent magnets, and the stator comprises coils and associated electrical circuitry which controls the switching of electrical power between the coils.
Such designs, being heteropolar devices, require the polarity of the electromagnets as formed by the coils, whether stationary or rotating, to be reversed as the motor turns.
It is an object of the subject invention to provide a DC machine in the form of a DC motor which operates without the xe2x80x9ccoggingxe2x80x9d effect commonly known to commutated DC motors.
It is another object of the subject invention to provide a DC machine in the form of a DC motor which operates without the eddy current losses commonly known to commutated DC motors.
It is a further object of the subject invention to provide a DC machine in the form of a DC motor which operates without the hysterisis losses commonly known to commutated DC motors.
It is a further object of the subject invention to provide a DC machine in the form of a DC motor which has only one air gap, instead of the two air gaps commonly known to commutated heteropolar DC motors.
It is a further object of the subject invention to provide a DC machine in the form of a DC motor which includes only a small number of separate components which may be manufactured and assembled by techniques adapted to industrial-scale manufacture, thereby resulting in a machine which within the scope of manufacturing tolerances exhibits high performance at the lowest possible production cost.
The invention seeks to provide a DC machine and in particular a DC motor having an unprecedented construction.
According to the invention, there is provided a DC machine for use as a DC motor or generator, comprising an outer casing, and a rotor comprising a central shaft and at least one cylindrical permanent magnet which has a central axis coincident with the shaft and is supported for rotation about its axis within the casing. The magnet provides a unipolar radial magnetic field. An annular gap is formed within the rotor, which extends co-axially with the magnet such that the magnetic flux of the radial magnetic field extends at substantially right angles across opposite sides of the gap. The machine includes a cylindrical stator located co-axially within the gap. The stator comprises an armature in the form of a hollow cylindrical roll of elongate insulating substrate having one surface bearing a conductive pattern. The pattern has two ends for connection to a DC power source and includes a multiplicity of straight conductors. The conductors extend axially with respect to the roll and are interconnected such that a DC current can flow along the conductors in the same axial direction for intersection with the magnetic flux of the radial magnetic field to cause rotation of the rotor.
Preferably, the rotor includes a ferromagnetic core through which the shaft extends, and the magnet is located by the core on one side of the gap.
More preferably, the core comprises a tubular central portion, a cylindrical sleeve surrounding the central portion in a concentric manner, and a circular connecting portion integrally interconnecting adjacent ends of the central portion and sleeve, said central portion and sleeve defining a cylindrical gap between them including the annular gap.
Further more preferably, the core includes an annular ball race which is located co-axially between the central portion and the sleeve adjacent the connecting portion and supports an inner end of the stator.
In a preferred embodiment, the rotor includes a ferromagnetic core through which the shaft extends, and two said magnets are located by the core, one co-axially inside the other and forming the gap therebetween.
In a specific construction, the magnet is formed by a plurality of magnet segments arranged in a circle.
It is preferred that the substrate is laminated, on said surface, with a layer of conductive material having parts thereof removed to form the conductive pattern.
In a preferred embodiment, the straight conductors are connected in parallel in the conductive pattern.
More preferably, the straight conductors are divided in sequential groups which are connected in series, within each group the straight conductors are connected in parallel.
Further more preferably, the number of straight conductors in each group is the same as the other groups.
It is preferred that the parts of the substrate over which the groups of straight conductors lie are impregnated with a particulate ferromagnetic material.
It is preferred that the groups of straight conductors in each layer, while the substrate is rolled up, are arranged to be substantially aligned with the groups in an adjacent layer.
It is preferred that the conductive pattern includes a segment connecting each group of straight conductors to an adjacent group, said segment being inclined at an acute angle to the straight conductors.
More preferably, the acute angle is substantially 45xc2x0.
It is an advantage that the total cross-sectional area of the straight conductors in each group is substantially the same as the cross-sectional area of each segment.
Preferably, the segments in each layer, while the substrate is rolled up, are arranged to be substantially aligned with the segments in an adjacent layer, and the aligned segments are shielded by a magnetic field shading material
Preferably, the groups of straight conductors and the alternating segments together resemble a waveform.
More preferably, the groups of straight conductors are arranged at gradually changing distance apart from one end of the substrate to the other end, such that when the substrate is rolled up from said one end the groups in each layer are substantially aligned with the groups in an adjacent layer.